1. Field of the Invention
The present disclosure relates to a method and system for measuring a concentration of a non-aqueous electrolyte in an eletrowinning process of metal and pyroprocessing of spent nuclear fuel.
2. Background of the Invention
Spent nuclear fuel made in a nuclear power plant includes a significant amount of high-level wastes as well as a large amount of unreactive uranium. In order to recycle unreacted uranium and reduce the volume of high-level wastes, recently, pyroprocessing has been highlighted. Pyroprocessing includes an electrolytic-reduction process, an electrolytic-refining process, and an electrowinning process. The crux of these processes is melting spent nuclear fuel in molten salt, electrochemically reducing respective elements such as uranium, trans-uranium, and the like, and electrodepositing and recovering the same on a cathode.
The pyroprocessing recovers uranium and trans-uranium (TRU) elements from molten salts, so it is critical to accurately measure components and concentration of uranium and trans-uranium elements existing in molten salt in carrying out the respective processes such as electrolyte refining, electrowinning Conventionally, components and concentration of uranium and trans-uranium elements are measured by sampling (or collecting) molten salt by using an analysis technique such as inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectrometer (ICP-MS), but in order to smoothly carry out the process, real-time monitoring solute concentration of molten salt having a relatively high temperature on the spot is urgently required.
Real-time monitoring method required for pyroprocessing may be classified into two fields.
A first method is a spectroscopic measurement method devised to allow absorption spectroscopy, laser-induced fluorescence spectroscopy, or the like, to be applied to pyroprocessing. The spectroscopic method mainly uses a light source, a measurement cell, and a detector, and has an advantage in that a characteristic signal according to an element can be detected, but disadvantageous in that it is difficult to install these components in a molten salt cell having a high temperature of pyroprocessing.
A second measurement is a method of applying an electrochemical measurement method to real-time monitoring. This measurement method is a method of measuring a type and amount of a solute existing in molten salt having a high temperature by measuring an electrochemical signal, i.e., a current signal. Since a process system is configured as an electrochemical system, the electrochemical measurement method is expected to be easily applied to a process cell. Electrochemical measurement methods known so far include a normal pulse voltammetry, a square wave voltammetry, and the like. With these methods, when a reaction of a solute existing in an electrode/solution interface is measured by repeating electrodeposition and dissolution reactions in a manner of scanning while applying pulses, the solute existing in a bulk of the solution is drawn to the electrode interface due to a diffusing phenomenon to increase concentration of the solute to the electrode/solution interface, having shortcomings in that a proportional relationship is not obtained in high concentration of 4 wt % or higher. Also, in case of using a conventional cyclic voltammetry or chronoamperomery, an excessive amount of electrodeposits are formed simultaneously with an electrodeposition reaction to rapidly increase an electrode area, having a problem in measuring a solute of high concentration. Besides, with the measurement methods, rough concentration of spent nuclear fuel in the molten salt having a high temperature can be measured, but since a potential scan time is protracted, lengthening a measurement time, and reduction potential is not appropriate for analyzing neighboring elements. In addition, molten salt having a high temperature is in a very highly harsh environment, so many materials are not stable in the molten salt having a high temperature, and when an electrode is put in the molten salt having a high temperature, a surface of the molten salt having a high temperature rises, changing a contact area of the electrode. In order to constantly maintain an electrode area, a method of wraping (or covering) the electrode with an insulator may be used, but when a potential of a reduced area is applied to the alumina used as an insulator within the molten salt having a high temperature, the alumina, or the like, is reduced to an aluminum metal so as to be changed into a conductor in quality, disadvantageously increasing the electrode area. Besides, in performing an electrochemical measurement on the solute of high concentration in the non-aqueous solution, if the electrode area is large, a middle portion of the electrode is cavitated of solute ions during the electrochemical measurement. Thus, the electrochemical measurement of the solute of high concentration by using an electrode having a large area cause a certain area of the electrode to be incapacitated, resulting in that a value lower than a current proportional to high concentration is measured.
Also, concentration measurements of oxygen anions have been performed by using an electrochemical method such as cyclic voltammetry, square wave voltammetry (SWV), or the like. These methods use a reaction of oxidizing oxygen anions to evolve oxygen. However, while a reaction current is being measured, oxygen continues to be produced, and to cling to an electrode surface, which induces the electrode area decreasing.
As a process of refining metal ores existing in nature, electrolytic refining process is commonly performed in a non-aqueous solution. Metal ores include a metal oxide as a combination of metal and oxygen, and during an electrolytic refining process, metal cations are reduced at a cathode to electroseparate metal so as to be recovered, and oxygen anions are oxidized at an anode to generate oxygen. In this process, the metal electrolytic reduction reaction occurs based on a principle similar to that of the pyroprocessing. Thus, the metal ore refining process also urgently requires a real-time measurement method for measuring elements refined in a non-aqueous solution in real time, in order to monitor an ongoing situation of the process.